remove ununsed deprecated field

[chunhtai]

Description

This deprecated field is added when we were doing soft-transition a fix into our internal code. The migration is completed, and the flag is no longer needed

Related Issues

Fixes #59316

Tests

I added the following tests:

No Test needed

Checklist

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Breaking Change

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[flutter-dashboard]

It looks like this pull request may not have tests. Please make sure to add tests before merging. If you need an exemption to this rule, contact Hixie on the #hackers channel in Chat.

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[goderbauer]

LGTM